(I)
an=a1.q^(n-1)
a4+a5 =6a3
a1q^3.(1+q)= 6a1.q^2
q(1+q)= 6
q^2+q-6=0
(q+3)(q-2)=0
q=2
a2=1/4
a1.q =1/4
2a1=1/4
a1=1/8
an =(1/8).2^(n-1) = 2^(n-4)
(II)
bn
=log<2>an
=log<2> 2^(n-4)
=n-4
bn <0
n-4 <0
n<4
Sn =b1+b2+...+bn
min Sn = S3 or S4 = -3-2-1= -6
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